In recent years, in consideration of a global environment, a reduction in carbon dioxide (CO2) emissions has been desired. Therefore, enhancements of lean burn operation are being made in order to achieve better fuel consumption of an internal combustion engine of a vehicle. Examples of a lean burn engine include a lean-burn gasoline engine, a direct-injection engine and a diesel engine. However, a conventional three-way catalyst cannot reduce and purify nitrogen oxide (NOx) effectively since exhaust gas emitted from the above-mentioned engines contains a lot of oxygen. Therefore, advances are being made in the development in the effective purification of the exhaust gas in various ways.
One of effective methods to purify the exhaust gas is to use a NOx trap catalyst. The NOx trap catalyst oxidizes and traps NOx in the exhaust gas when an air-fuel ratio is lean, while the NOx trap catalyst releases the trapped NOx to reduce to nitrogen (N2) when the air-fuel ratio is stoichiometric or rich. In such a case, the NOx trap catalyst reduces the NOx by increasing a reducing agent (hydrogen (H2), carbon monoxide (CO), hydrocarbon (HC)) in the exhaust gas. However, the excessive reducing agent, especially excessive hydrocarbon, is released without used for the NOx reduction, and the released reducing agent reacts with oxygen, which may cause the emission amount of CO2 to increase. Further, the rapid shift of the air-fuel ratio of the exhaust gas to stoichiometric or rich in order to increase the reducing agent in the exhaust gas unfavorably causes degradation of driving performance and fuel consumption.
In view of this, attempts to use more effective reducing agents, especially hydrogen, are being developed with respect to the NOx reduction. In addition, a catalyst that generates hydrogen by steam reforming has been proposed (for example, refer to Patent Document 1).